he universe doesnt abide by what you see is what you get In fact the stuff we see in space - stars gas and dust - accounts for only 10 percent of the universes mass

This visible stuff is ordinary matter and its made up of protons neutrons and electrons Scientists call ordinary matter baryonic matter because protons and neutrons are subatomic particles called baryons The other 90 percent of the mass is dark matter and it likely surrounds almost every galaxy in the universe

Dark matter doesnt emit absorb or reflect any type of light (so for example

Liz Kruesi is an associate editor of Astronomy

28 Astronomymiddot November 09

it doesnt emit X-rays or absorb infrared radiation) This mysterious stuff is thereshyfore invisible yet astronomers learned it exists because dark matter interacts with ordinary matter through gravity

Searching in the dark Swiss astrophysicist Fritz Zwicky first proposed dark matters existence in 1933 While studying the Coma cluster of galshyaxies he found that the galaxies collecshytive gravity alone was much too small to hold the cluster together

The next round of evidence came in the 1970s Astronomers charted the velocities of stars at various distances from the center of a spiral galaxy and

plotted the velocity versus the distance to create a rotation curve They expected the velocities to reach a maximum and then decrease farther from the center shybut the data showed otherwise The velocities reach a maximum and then plateau With velocities so high at the outer edge of galaxies the stars should fling out of their orbits But they dont Some sort of mass scientists cant detect must be holding these outer stars in orbit

A very massive object - such as a galshyaxy cluster - can act as a gravitational lens Some images of regions around galshyaxy clusters show numerous arcs Those are background galaxies distorted and magnified by the clusters gravity

Astronomers study the sizes and shapes of those arcs to determine a clusshyters mass By comparing that calculated mass to the mass that comes from only luminous objects (the galaxies) astronoshymers can determine how much dark matter is in a cluster

Other evidence has turned up in collishysions of galaxy clusters namely the Bullet cluster This object is actually the aftershymath of two galaxy clusters that collided Astronomers used a multidetection approach to look at the galaxies gas and dark matter When the clusters coUided the galaxies stars passed through mostly unaffected because a lot of space exists between the stars The clusters hot gas

makes up most of their baryonic mass Ordinary matter interacts through elecshytromagnetic forces Thus as matter colshylides it loses energy as radiation (in the form of X-rays in this case) The hot gas slows during the collision

Astronomers use gravitational lensing to indirectly map the dark matter distrishybution It turns out that dark matter also passed through the collision unaffected So images of the Bullet cluster show direct evidence of dark matter

The evidence is piling up - with 3-D dark matter maps and other detections Yet mapping the distribution is one thing knowing the characteristics of this mysteshyrious stuff is another story

Unlike anything weve seen For many years astronomers thought dark matter could consist of dead stars black holes and other known objects that emit little or no light They used gravitational microlensing to look for these objects This technique is similar to gravitational lensing except the foreground object is much less massive Instead ofIight bendshying around the object the bodys gravity magnifies the light from behind it While astronomers found some of these MAssive Compact Halo Objects (MACHOs) there werent enough to account for all of the universes missing mass

So if dark matter isnt composed of normal objects then it likely consists of

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Astronomers study the sizes and shapes of those arcs to determine a clusshyters mass By comparing that calculated mass to the mass that comes from only luminous objects (the galaxies) astronoshymers can determine how much dark matter is in a cluster

Other evidence has turned up in collishysions of galaxy clusters namely the Bullet cluster This object is actually the aftershymath of two galaxy clusters that collided Astronomers used a multidetection approach to look at the galaxies gas and dark matter When the clusters coUided the galaxies stars passed through mostly unaffected because a lot of space exists between the stars The clusters hot gas

makes up most of their baryonic mass Ordinary matter interacts through elecshytromagnetic forces Thus as matter colshylides it loses energy as radiation (in the form of X-rays in this case) The hot gas slows during the collision

Astronomers use gravitational lensing to indirectly map the dark matter distrishybution It turns out that dark matter also passed through the collision unaffected So images of the Bullet cluster show direct evidence of dark matter

The evidence is piling up - with 3-D dark matter maps and other detections Yet mapping the distribution is one thing knowing the characteristics of this mysteshyrious stuff is another story

Unlike anything weve seen For many years astronomers thought dark matter could consist of dead stars black holes and other known objects that emit little or no light They used gravitational microlensing to look for these objects This technique is similar to gravitational lensing except the foreground object is much less massive Instead ofIight bendshying around the object the bodys gravity magnifies the light from behind it While astronomers found some of these MAssive Compact Halo Objects (MACHOs) there werent enough to account for all of the universes missing mass

So if dark matter isnt composed of normal objects then it likely consists of

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Two massive galaxy dusters collided to form whats known as the Bullet cluster of galaxies Ordishynary matter - the hot gas shown in pink - collided lost energy and slowed The clusters dark matter (shown in blue) interacted little and passed through the ordinary matter

non-baryonic particles - meaning its not made up of the same stuff as ordinary matter (protons and neutrons) Astronoshymers split non-baryonic dark matter into two categories hot and cold These titles have nothing to do with temperature Hot means that early in the universe these particles traveled extremely fast shyalmost at the speed of light Cold means that early in the universe the particles traveled more slowly

How does particle speed relate to dark matters composition Slower particles will bunch up into small structures earshylier in the universe Those small strucshytures will eventually collide and merge to form larger ones Astronomers believe this is how structure develops and evolves in our universe Smaller strucshytures eventually merge into the massive superclusters we observe today Astronoshymers simulate structure evolution with cold dark matter (CDM) and can create models that resemble todays universe

What is CDM Scientists arent sure yet They have a couple of options that branch from particle physics - but none contrived just to fit into dark matter theshyories Both [options 1are generated by particle theories having nothing to do

with dark matter says Juan Collar of the University of Chicago However these hypothetical particles turn out to have all of the properties (mass abundance lifeshytime probability and mode of interacshytion) required to be the dark matter or at least a fraction of it

For decades physicists have worked to explain how the four fundamental physishycal forces fit together (These forces are gravitation electromagnetism weak nuclear and strong nuclear) In the past 30 or so years theyve arrived at supershysymmetry theory This model predicts

Astronomers were surprised to find that stars far from a galaxys core travel at speeds similar to those close in John Smith

that each ordinary particle (such as an electron or quark) has a massive supershypartner (a selectron or squark) that remains undetected

The leading dark matter candidate is a class of particles that supersymmetry predicts These particles have mass and interact through the weak nuclear force but they dont interact through the elecshytromagnetic force Because these weakly interacting massive particles (WIMPs) interact via the weak force they can colshylide with normal atomic nuclei and bounce off them without emitting light or absorbing radiation The lightest WIMP - called the neutralino - is also the most popular dark matter possibility

Another common CDM candidate is the axion The axion is also a hypothetishycal particle but it arises from a theory different from supersymmetry This parshyticle is not a matter particle but instead a force carrier similar to the photon (which carries the electromagnetic force) Its much lighter than a WIMP shyat least 1 billion times less massive - so the universe would need a whole lot more axions than WIMPs to make up all the invisible matter

One would expect that with so many CDM particles WIMPs or axions would be easy to find But because they dont interact through the electromagnetic force detecting them pushes scientists experimental limits

How to hunt CDM The method used to detect dark matter depends on what type of dark matter (WIMP or arion) scientists pursue Scishyentists looking for WIMPs try to directly observe the interaction with ordinary

A galaxys rotation curve compares the disk stars velocities with their distances from the galaxys center amy Roen Kelly

30 Astronomymiddot November 09

The universes structure seems to evolve as smaller clumps bunch to form larger structures Astronomers simulate structure evolution with cold dark matter and create models that resemble todays universe This simulation shows dark matter distribution where brighter areas represent more dense regions

matter in a detector A WIMP can collide with an atomic nucleus and move or scatter the nucleus

Another method is to indirectly detect dark matter A WIMPs antiparticle is itself so if two WIMPs interact they annihilate each other and produce a shower of secondary particles Astroshyphysicists can observe many of these secshyondary particles - such as electrons positrons (the electrons antiparticle) gamma rays and neutrinos

Scientists methods to locate axions are totally different than the direct and indirect detection methods used to look for WIMPs says Dan Hooper of Fermi National Accelerator Laboratory in Batashyvia Illinois When an axion traverses a detector that has a magnetic field it will convert into a photon

Instead of trying to detect CDM parshyticles some scientists aim to create the particles - WIMPs and axions - in the laboratory To do this they have to genshyerate extremely high energies similar to those shortly after the Big Bang Only particle accelerators have this ability After the Large Hadron Collider (the worlds largest particle accelerator located in Switzerland) comes back online late this year scientists should be able to look for hypothetical particles that may make up dark matter

Bullying the WIMPs Astronomers believe a spherical halo of CDM surrounds the luminous galactic disk of the Milky Way (and similar halos encase most other galaxies) As our solar system travels around the galaxys center

Superduster Abell 901902 ltontains hundreds of galaxies Astronomers analyzed the distortion of some 60000 background galaxies shapes to determine the superclusters distribution of dark matshyter They then ltombined a visible-light image of the supercluster with the dark matter distribution map (shown as a magenta haze)

it moves through this dark matter haze These particles arent the only things Earth collides with as it moves through the haze Incoming high-energy ordinary particles called cosmic rays bombard Earth constantly Radiation from the Sun and more distant sources do too

Scientists seek the WIMPs that may compose the CDM haze by placing most dark matter detectors underground and shielding th em to block the detector material from cosmic rays The key is to be able to block signals from backshyground noise and detect when a dark matter particle interacts with the mateshy

rial And if they cant block all of the noise they must be able to tell the differshyence between noise and a WIMP

Some scientists think about 600 milshylion WIMPs pass through a square meter of Earths surface every second But remember that they interact weakly So how do detectors see a WIMP During a rare collision the WIMP will transfer some of its energy to an atoms nucleus of the detector material and as a result the nucleus scatters (think of pool balls) The amount the nucleus moves (or recoils) is related to the WIMPs energy Scientists detect this recoil a few different ways

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The universes structure seems to evolve as smaller clumps bunch to form larger structures Astronomers simulate structure evolution with cold dark matter and create models that resemble todays universe This simulation shows dark matter distribution where brighter areas represent more dense regions

matter in a detector A WIMP can collide with an atomic nucleus and move or scatter the nucleus

Another method is to indirectly detect dark matter A WIMPs antiparticle is itself so if two WIMPs interact they annihilate each other and produce a shower of secondary particles Astroshyphysicists can observe many of these secshyondary particles - such as electrons positrons (the electrons antiparticle) gamma rays and neutrinos

Scientists methods to locate axions are totally different than the direct and indirect detection methods used to look for WIMPs says Dan Hooper of Fermi National Accelerator Laboratory in Batashyvia Illinois When an axion traverses a detector that has a magnetic field it will convert into a photon

Instead of trying to detect CDM parshyticles some scientists aim to create the particles - WIMPs and axions - in the laboratory To do this they have to genshyerate extremely high energies similar to those shortly after the Big Bang Only particle accelerators have this ability After the Large Hadron Collider (the worlds largest particle accelerator located in Switzerland) comes back online late this year scientists should be able to look for hypothetical particles that may make up dark matter

Bullying the WIMPs Astronomers believe a spherical halo of CDM surrounds the luminous galactic disk of the Milky Way (and similar halos encase most other galaxies) As our solar system travels around the galaxys center

Superduster Abell 901902 ltontains hundreds of galaxies Astronomers analyzed the distortion of some 60000 background galaxies shapes to determine the superclusters distribution of dark matshyter They then ltombined a visible-light image of the supercluster with the dark matter distribution map (shown as a magenta haze)

it moves through this dark matter haze These particles arent the only things Earth collides with as it moves through the haze Incoming high-energy ordinary particles called cosmic rays bombard Earth constantly Radiation from the Sun and more distant sources do too

Scientists seek the WIMPs that may compose the CDM haze by placing most dark matter detectors underground and shielding th em to block the detector material from cosmic rays The key is to be able to block signals from backshyground noise and detect when a dark matter particle interacts with the mateshy

rial And if they cant block all of the noise they must be able to tell the differshyence between noise and a WIMP

Some scientists think about 600 milshylion WIMPs pass through a square meter of Earths surface every second But remember that they interact weakly So how do detectors see a WIMP During a rare collision the WIMP will transfer some of its energy to an atoms nucleus of the detector material and as a result the nucleus scatters (think of pool balls) The amount the nucleus moves (or recoils) is related to the WIMPs energy Scientists detect this recoil a few different ways

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One type of detector uses crystals kept at frigid temperatures (only 001 degree above absolute zero) Crystals have a set structure so when a WIMP collides with an atomic nucleus the nucleus recoils and rams into surrounding structure In these collisions the scattered nucleus transfers some of its kinetic energy and slightly heats the material The frigidity ensures that the detected vibrations have resulted from only incoming particle interactions Of course the scientists will likely detect particles other than just WIMPs So most WIMP detectors use multiple methods to determine on an event-by-event basis if what took place looked like a dark matter particle interaction or something more mundane says Collar

When a WIMP scatters the atomic nucleus and hits surrounding atoms it could knock off electrons therefore ionshyizing these atoms Certain ionization detectors can measure these loose charges

In some materials such as liquid xenon a light flash will indicate a WIMP After the scattered nucleus rams into other atoms and frees electrons the atom emits a light flash called scintillation Usually if a detector looks for scintillashytion it will also hunt for ionization

Another approach to the direct search is using a bubble chamber - a glass jar filled with a specific type of liquid When a WIMP hits an atomic nucleus it will produce a tiny bubble Scientists then watch the bubble grow How it grows depends on whether the interactshying particle was a WIMP or a backshyground particle

A reliable WIMP detection would be if the WIMP signal varied as a result of the time of year This is because Earth revolves around the Sun In June Earths movement is in the same direction as our solar systems path around the galaxy so the detected signal should increase In December Earth moves in the opposite direction and scientists should detect a signal some 5 to 10 percent smaller This signal difference helps distinguish the WIMPs from the background noise because the noise remains the same while the WIMP signal modulates

The team of scientists with the DArk MAtter (DAMA) experiment claimed some years ago (and again in 2008) that it found evidence for the existence of WIMPs by looking at this modulation Unfortunately DAMA used only one detection method and therefore may not have been able to discriminate between background noise and a WIMP signal And no other scientific group has repeated DAMis discovery In science if another group cant repeat a fmding then theres a distinct possibility that experimental error and not evidence is responsible

WIMPy signals So far direct searches havent found WIMPs Therefore scientists also look for the indirect signature of the dark matter candidates to complement direct searches Neutralino annihilations should produce electrons positrons gamma rays and neutrinos along with other particles Scishyentists can use certain detectors to look for each product

The density of neutralinos (or other WIMPs) must be high in order for these particles to meet up and destroy each other This typically happens within more massive objects

A WIMP near the Sun or Earth could collide with an ordinary particles nucleus (This is similar to what happens within detector material) The WIMP will lose energy and its speed could decrease below the Sun or Earths escape velocity If that happens the WIMP cannot escape the massive objects gravitational hold The WIMP can collide with another nucleus and so on until it settles into the core of either the Sun or Earth

At the core the densities are so high that WIMPs collide and produce secondshyary particles and radiation (As menshytioned before neutrinos and gamma rays are two such products) Several experishyments underground - such as SupershyKamiokande in Japan - detect neutrinos

WIMP collisions arent the only nearby events that release neutrinos - the Sun produces them Neutrino detectors can deCipher WIMP neutrinos from solar neutrinos because WIMP neutrinos have

greater energies And a larger detector should find more neutrinos (hopefully of the WIMP variety) The next-generation neutrino detector IceCube should help in this search IceCube is currently being built at the South Pole and will cover a very large area - a cubic kilometer

Searches for gamma rays from WIMP annihilations also look promising The gamma rays should have a specific energy spectrum that depends on how massive the WIMP is The Fermi Gamma-ray Space Telescope may be able to detect that particular spectrum and offer an indirect observation of dark matshyter Says Hooper If I had to wager a guess I would say that the best prospects to detect WIMPs in the near future are with gamma-ray telescopes A number of ground-based gamma-ray detectors are also on the lookout

Wheres the axion A WIMP may be the leading CDM candishydate but it isnt the only one The axion is also a popular possibility

An axion detector consists of two parts a cavity with a magnetic field and

an antenna with amplifier According to theory as an axion traverses the cavity it will convert into a microwave photon The photons frequency will be proporshytional to the axions mass Scientists however arent sure what the axions mass is which means theyre unsure what freshyquency to search for Using the antenna and amplifier scientists will scan porshytions of the microwave region looking for a signal that stands out above the backshyground noise

Detector sensitivity is slowly getting to where it needs to be to pick out axions - and WIMPs - from background noise Its not there yet but scientists with the help of elegant particle theories are throwing everything they have at searching for (and hopefully detecting) dark matter

Very often in particle physics we have followed such natural prescriptions only to be surprised by nature says Colshylar With more advanced detectors comshying in the next decade or so cosmologists are sure to get a surprise - whether a hint that theyre on the wrong path or a promising detection ~

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The density of neutralinos (or other WIMPs) must be high in order for these particles to meet up and destroy each other This typically happens within more massive objects

A WIMP near the Sun or Earth could collide with an ordinary particles nucleus (This is similar to what happens within detector material) The WIMP will lose energy and its speed could decrease below the Sun or Earths escape velocity If that happens the WIMP cannot escape the massive objects gravitational hold The WIMP can collide with another nucleus and so on until it settles into the core of either the Sun or Earth

At the core the densities are so high that WIMPs collide and produce secondshyary particles and radiation (As menshytioned before neutrinos and gamma rays are two such products) Several experishyments underground - such as SupershyKamiokande in Japan - detect neutrinos

WIMP collisions arent the only nearby events that release neutrinos - the Sun produces them Neutrino detectors can deCipher WIMP neutrinos from solar neutrinos because WIMP neutrinos have

greater energies And a larger detector should find more neutrinos (hopefully of the WIMP variety) The next-generation neutrino detector IceCube should help in this search IceCube is currently being built at the South Pole and will cover a very large area - a cubic kilometer

Searches for gamma rays from WIMP annihilations also look promising The gamma rays should have a specific energy spectrum that depends on how massive the WIMP is The Fermi Gamma-ray Space Telescope may be able to detect that particular spectrum and offer an indirect observation of dark matshyter Says Hooper If I had to wager a guess I would say that the best prospects to detect WIMPs in the near future are with gamma-ray telescopes A number of ground-based gamma-ray detectors are also on the lookout

Wheres the axion A WIMP may be the leading CDM candishydate but it isnt the only one The axion is also a popular possibility

An axion detector consists of two parts a cavity with a magnetic field and

an antenna with amplifier According to theory as an axion traverses the cavity it will convert into a microwave photon The photons frequency will be proporshytional to the axions mass Scientists however arent sure what the axions mass is which means theyre unsure what freshyquency to search for Using the antenna and amplifier scientists will scan porshytions of the microwave region looking for a signal that stands out above the backshyground noise

Detector sensitivity is slowly getting to where it needs to be to pick out axions - and WIMPs - from background noise Its not there yet but scientists with the help of elegant particle theories are throwing everything they have at searching for (and hopefully detecting) dark matter

Very often in particle physics we have followed such natural prescriptions only to be surprised by nature says Colshylar With more advanced detectors comshying in the next decade or so cosmologists are sure to get a surprise - whether a hint that theyre on the wrong path or a promising detection ~

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